1. Field of the Invention
The present invention relates to a fuel injection control system for an internal combustion engine. Specifically, the present invention relates to a method for cleaning exhaust gas.
2. Description of Related Art
Improvement in exhaust emission of an internal combustion engine, which is mounted on a vehicle and the like, is required. Specifically, exhaust particulate matters such as soot or a soluble organic fraction included in the exhaust gas discharged from a compression-ignition type diesel engine running on diesel oil should be removed, as well as carbon oxide, hydrocarbon and nitrogen oxide. Therefore, a particulate filter made of a porous material is disposed in an exhaust passage of the diesel engine for collecting the exhaust particulate matters included in the exhaust gas.
Since the particulate filter is made of a porous material, flow resistance at the particulate filter may increase if an excessive quantity of the particulate matters is collected. As a result, back pressure of the engine may be increased, and the engine output may be decreased. Therefore, ability of the particulate filter to collect the exhaust particulate matters should be recovered by regenerating the particulate filter. The particulate filter is regenerated by eliminating the collected exhaust particulate matters.
A particulate filter having an oxidation catalyst such as platinum thereon can be regenerated during the operation of the engine with the use of an oxidization effect of the oxidization catalyst. For instance, a post-injection for injecting fuel in an expansion stroke or an exhaustion stroke of the engine is performed to provide the fuel to the particulate filter. Thus, the fuel is oxidized by the oxidization catalyst, generating reaction heat. Then, the particulate matters collected by the particulate filter are combusted and eliminated with the use of the heat. In the case in which the oxidization catalyst is supported on the porous material of the particulate filter, the particulate filter is heated up to a temperature suitable for combusting the exhaust particulate matters, which are more difficult to combust than normal fuel such as injection fuel.
Oxidization ability of the oxidization catalyst depends on the temperature. Therefore, there is a possibility that the oxidization ability of the catalyst may become insufficient even if a theoretical quantity of the fuel corresponding to the heating value required for eliminating the collected particulate matters is injected in the post-injection. As a result, white smoke, or an unburned fraction of the fuel, may be generated.